The present invention relates to the field of signal transmission on telephone lines and in particular to a procedure and device for adaptive digital cancellation of the echo generated in non-constant telephone connections.
It is known that in long distance telephone connections there can occur voice signal reflections toward the same subscriber who generated them. This is due mainly to the impedance mismatch of the hybrid coils placed between the 4-wire lines and 2-wire lines to which lead the two telephones placed at the two ends of the connection. Because of the reflections, a subscriber, while he is still talking, listens to what he said with a delay depending on the length of the connection. The signal fraction listened to is the echo of the signal transmitted, and the path the echo completes, from the point of generation to the point of listening, is called an echo channel.
It has been proven that the effects of the echo are annoying for delays over a few tens of milliseconds, and echo channel attenuations between 6 dB and 15 dB. To obviate this drawback, in long distance telephone connections appropriate correction devices called echo cancellers are adopted.
These devices are generally inserted in 4-wire lines connected to an international exchange and suppress the echo produced during the conversation by subtracting from the actual echo signal an estimate thereof obtained by means of a digital filter with adaptive coefficients which again copies the pulse response of the echo channel.
In known echo cancellers there is also generally present a non-linear circuit which entirely eliminates the echo residue when it is less than a predetermined threshold. In addition they are equipped with a double-talking detector circuit which, in the presence of speech of a near-end talker, inhibits both updating of the transverse filter coefficients and operation of the non-linear circuit so that it does not attenuate the speech.
As is known, the echo channel can have a transfer function which is not constant in time. In this case its pulse response becomes a function variable in time and can significantly influence the performance of the canceller.
This phenomenon occurs in FDM-SSB-SC systems with a single physical communication path which transmit in both directions using carriers with different frequencies.
The modulation and demodulation frequencies of the carrier system are not synchronized, being locally generated. This can cause translation of all the echo signal frequency components with respect to the signal sent. This phenomenon, called phase-roll, causes the non-constant nature of the echo channel.
International guide-lines in this matter are set for example by CCITT G.311 recommendations which require that the frequency shift between the two ends of a connection must not exceed 2 Hz in each direction. Consequently the upper limit for total frequency shift on the echo path is 4 Hz.
A first known echo canceller intended to solve the problems of echo cancellation in connections of any distance affected by phase-roll is the one based on a procedure described in the Italian patent application no. 68047-A/83, inventors R. Montagna and L. Nebbia, filed in the name of CSELT on 12 Oct. 1983 (corresponding to U.S. Pat. 4,736,414). This canceller benefits from the fact that the pulse responses of the echo channels are characterized by a pure propagation delay followed by a zone approximately 6 ms wide in which the pulse response is significantly different from zero. Consequently, in accordance with the above procedure, evaluation is made of the signal propagation delay, for the purpose of adopting a short transverse filter whose coefficients correspond only to the significant part of the pulsed response. The filter used is the non-recursive type (FIR) with adaptive coefficients updated by the gradient method.
The limited number of coefficients to update makes the canceller faster in producing the echo estimate, allowing its use even in connections affected by limited phase-roll.
Even though the canceller mentioned above has better performance than the previous ones in connections affected by phase roll, it is not capable of maintaining good operational performance in the presence of sustained phase roll, e.g. on the order of 4 Hz. A second known echo canceller is the one described in the text of Italian patent no. 1228106, inventors Giacomo Cannalire, Giacomo Premoli and Roberto Ravasio,issued 28 May 1991 and filed 21 Dec. 1988 in the name of the same applicant (corresponding to U.S. Ser. No. 07/870,179 filed Apr. 16, 1992). The canceller described therein allows for the echo signal propagation delay to limit the pulse response estimate of the echo channel to only the significant part thereof, similarly to what was done in the Nebbia-Montagna canceller, differentiating itself from it and from the majority of known echo cancellers by the fact that it uses a recursive digital filter of the IIR type with adaptive coefficients. The latter are updated by means of an algorithm obtained by innovatively simplifying the conventional Kalman algorithm. As known,the Kalman algorithm possesses a considerable convergence speed but requires a considerable number of arithmetic operations. The innovative simplification was to divide the calculations for a generic iteration of the algorithm on several sampling intervals with a criterion called square error decimation. Decimation reduces the algorithm convergence speed, which however remains more than sufficient to compensate for a 4 Hz phase roll but also allows reduction of the number of arithmetic operations assigned to each sampling interval.
In comparison with earlier ones, this canceller shows clearly superior performance in connections affected by phase-roll up to 4 Hz. On the other hand, the complexity of the calculation due to the greater complexity of Kalman's algorithm compared with the gradient method is still considerable.